Override job `command` on the command line

[dnephin]

Sometimes it might be convenient to start an adhoc job with a different command. A common example might be starting an interactive shell in a container.

Along with #90 this might remove the need to provide a shell job for each image, if you want to start an interactive shell for debugging or development.

[Fuco1]

My current image I'm using has an ENTRYPOINT as Rscript start.R so I expected the command would just be attached to that as usual with docker run, i.e. in my dobi.yaml I wrote just

...
  command: "foo"
...

and I expected that Rscript start.R foo would execute. Further, passing arguments such as input file names to process would be convenient.

I'm not sure what impact this would have to caching.

Edit: this is in response to you saying

this might remove the need to provide a shell job for each image

I think it would be a good addition and the most expected scenario for a docker user.